Automatic control of band width in band-pass filters



8- 1940- R. RECHNITZER 2.210381 AUTOMATIC CONTROL OF BAND WIDTH IN BAND-PASS FILTERS Filed March 5, 1937 CURRENT-DE ENDEAIT RES/STANCE COPPER- OX/DE RECTIFIER) INVENTOR R'UDOLF RE HNITZER ATTORNEY Patented Aug. 6, 1940 PATENT OFFICE A AUTOMATIC co-N'mor. F BAND WIDTH IN BAND-PASS FILTERS Rudolf Rechnitzer, Budapest,

to Telefunken Gesellschaft graphic, in. b. H., Berlin,

tion of Germany Hungary, assignor fur Drahtlose Tele- Germany, a corpora- Application March 3, 1937, Serial No. 128,698 In Germany March 4, 1936 V 2 Claims.

Regulation or control of the bandwidth and thus of selectivity is desirable in receiver apparatus. With this end in view it is usual to change the damping or the coupling, or both 5 simultaneously. By mechanical means such a change or regulation is readily and easily feasible, though it is more desirable to attain the identical end by electrical means in order that regulation of selectivity may be accomplished automatically, for instance; as a function of the input amplitude or the amplitude or level of stray or noise.

Ways and means adapted to change the damping attenuation electrically are known in the prior art. For this purpose a damping or attenuator tube has been connected in parallel relation to the oscillation circuit. This invention discloses an electric method designed to insure a change in coupling. According to the invention a change in coupling is efiected by a currentdependent resistance which varies a coupling impedance.

The invention shall be explained in more detail by reference to the accompanying drawing wherein Figs. 1 2.11112 show different modifications of interstage band-pass filters according to the invention and where the coupler impedance referred to consists in each case of a capacity.

Referring to Fig. 1 M denotes an inductive coupling of the two oscillation circuits A and B connected respectively to the output of tube I and to the input of tube 2. The inductive coupling is in a sense such that it counter-acts the capacitive coupling constituted by the two condensers Ck. One of these two condensers is bridged or shunted by a current-dependent resistance, say, a copper-oxide type rectifier. According to the intensity of the radio frequency current flowing through the said resistance R, the value of the resistance will be changed with the result that the shunted capacity Ck will be more or less active. amplitude grows, the current will be caused to As the radio frequency.

increase in value and as a result the resistance R will drop with an incidental growth of the coupling capacity. The usefulness of this circuit scheme is dependent upon the application of a high radio frequency potential across the oscillation circuits. In order that the coupling capacities may not have to be chosen too low they may be applied in well known manner to taps on the inductances of the oscillation circuits A and B.

In Fig. 2 there is shown a circuit organization in which an auxiliary direct current serves for insuring a change in the coupling rather than the radio frequency current itself. The direct current component of the plate current of tube I flows across the resistance 7'. The fall of potential occasioned across this resistance results in a current which flows through the resistance R, and the auxiliary resistance H which is high in contrast to the coupling capacity. With a view to regulating the selectivity manually, resistance It may be made variable. In addition, however, there occurs an automatic regulation if the grid potential of tube l is adjusted for the purpose of insuring fading compensation or volume control. As a result there is also changed the plate current flowing through the resistance 1'.

What I claim is:

1. An interstage coupling circuit comprising a pair of tuned circuits, inductive coupling between said circuits, a pair of series-connected condensers connected between high potential points of said circuits 'to provide capacitive coupling, and an element whose resistance value varies with the current flow through it connected in shunt to each of the series-connected coupling condensers.

2. An interstage coupling circuit as defined in claim 1 wherein a second resistance element similar to the first is connected between the low potential terminals of the tuned circuits.

RUDOLF RECI-INITZER. 

